This invention relates generally to accumulators and in particular to an accumulator, used in an airborne, hydraulically operated integrated drive generator (IDG), for supplying hydraulic fluid during negative or zero xe2x80x9cgxe2x80x9d conditions.
An integrated drive generator, (IDG), is an integral unit having a constant speed drive continuously variable transmission (CVT) and an electric generator in the same housing. The IDG converts variable speed rotary input from a shaft of an aircraft propulsion engine into a constant speed shaft drive which drives the electrical generator typically producing constant frequency three phase 400 Hz power. The transmission can be any of a plurality of continuously variable transmissions such as a hydrostatic pump/motor assembly, a traction drive, or pulley varidrive.
Referring to FIG. 1, a typical IDG 10 includes a pressurized casing 12 which acts as an oil sump by collecting hydraulic fluid 26 at its bottom. To maintain constant speed control of the CVT, oil from the sump is pumped to the CVT controls. In a negative xe2x80x9cgxe2x80x9d condition, the oil is forced from the bottom of the sump to the top and in a zero xe2x80x9cgxe2x80x9d condition the oil floats in the middle between the top and the bottom. Both these conditions are referred to as adverse xe2x80x9cgxe2x80x9d conditions and when either occurs, the oil quantity at the bottom of the sump drops and the flow to the CVT is interrupted. Such an interruption or drop in oil flow to the CVT will result in loss of speed control. This means that the CVT will be unable to hold a constant generator speed which will cause an automatic shut down of the IDG.
Commercial aircraft experience zero or negative xe2x80x9cgxe2x80x9d conditions under a variety of circumstances such as severe weather or emergency maneuvers. Clearly, when these circumstances occur it is important that the CVT continue to maintain constant generator speed, otherwise the aircraft will lose electric power. As a consequence, IDGs used on commercial aircraft are typically required to operate normally for a duration of 15 seconds of zero or negative xe2x80x9cgxe2x80x9d forces.
One method used to meet this 15 second requirement is to provide a second pump for pumping oil from the top of the sump when negative xe2x80x9cgxe2x80x9d is experienced. This method has had only limited success, because mounted in the sump are a plurality of rotating components which inhibit the flow of oil from the bottom to the top. Instead of the oil flowing smoothly to the top, it gets flung around the casing by these rotating components. Further, this method does not address the zero xe2x80x9cgxe2x80x9d conditions where oil tends to float in the middle of the sump.
Copending U.S. patent application Ser. No. 09/482,212, which is assigned to the assignee of this application, discloses an IDG hydraulic system that overcomes the disadvantages of the prior art systems by utilizing in flow series arrangement of a scavenge pump, a spool valve and a boost pump. When spool valve detects an interruption in the supply pressure to the boost pump due to an adverse xe2x80x9cgxe2x80x9d condition, it reconfigures the system to a closed loop system. In this mode, the oil returning from the CVT is re-circulated back to the boost pump instead of back to the scavenge pump. A small oil accumulator is used to make up for leakage in the system. Typically, these oil accumulators utilize a mechanical spring acting on a piston or a bladder acted upon by a charged gas volume. Both of these types of accumulators add cost and volume to the design.
Accordingly, there is a need for an improved piston type accumulator that does not require a spring or other mechanical actuator.
An object of the present invention is to provide a fluid accumulator for use in an hydraulic control system for an integrated drive generator having a constant speed drive variable transmission for providing hydraulic fluid during an adverse xe2x80x9cgxe2x80x9d event.
The present invention meets these objectives providing an IDG hydraulic system that utilizes in flow series arrangement of a scavenge pump, a spool valve and a boost pump. When spool valve detects an interruption in the supply pressure to the boost pump due to an adverse xe2x80x9cgxe2x80x9d condition, it reconfigures the system to a closed loop system. In this mode, the oil returning from the CVT is re-circulated back to the boost pump instead of back to the scavenge pump. Oil lost to leakage is replenished by an oil accumulator that uses the gas pressure in the IDG""s casing to expel fluid from the accumulator into the recirculating flow.
These and other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.